1. Field of the Invention
The present invention generally relates to the art of microelectronics, and more specifically to an electrical microcircuit structure and process for fabricating the same from ceramic tape. The structure has a desired non-planar shape, and is especially suitable for hybrid microcircuit technology.
2. Description of the Related Art
Fabrication of multilayer electronic structures for hybrid microcircuit technology and other applications may be classified into three general processes. The first uses a so-called thick film process wherein individual conductor and dielectric compositions in paste form are sequentially deposited on insulating substrates and then fired, one layer of material at a time, in order to build up a thick film, multilayer circuit. A common method for depositing these thick film pastes involves the use of a screen printing process for depositing layers of a dielectric paste on the substrate surface, over any conductive pattern thereon, and then subsequently firing the layers at a predetermined elevated temperature in order to build up a "thick film" of a preferred thickness.
This prior art thick film process provides good fixed registration or positional accuracy, and good dimensional stability by being fired directly on the substrate. The thick film layers are positionally secured and permanently referenced to the substrate. However, a disadvantage of the thick film process is that voids can be formed in the thick film dielectric material during the sequential printing and firing process. Another disadvantage of the thick film process is that the requirement for building up many multiple thick film layers in the more complex hybrid circuits results in an expensive process due to the number of individual processing steps involved.
A third disadvantage of the thick film approach is that the top bonding conductor traces are typically rough and/or rounded as a result of being printed over numerous levels of conductor and dielectric.
The second approach to the fabrication of hybrid microcircuits is the cofired ceramic process. This technology utilizes dielectric material formed into sheets having alumina as a main component. These insulating sheets are then either metallized to make a ground plane, signal plane, bonding plane, or the like, or they are formed with via holes and back filled with metallization to form interconnect layers. Individual sheets of tape are then stacked on each other, laminated together using a predetermined temperature and pressure, and then fired at a desired elevated temperature at which the material fuses or sinters. Where alumina is chosen for the insulating material, tungsten, molybdenum or molymanganese is typically used for metallization, and the part is fired to about 1,600.degree. C. in an H.sub.2 reducing atmosphere. An alternative approach to using these base metal conductors is to use platinum or palladium metal and fire in air. These metals, however, are very costly and are rarely used.
The undesirable high processing temperature and requisite H.sub.2 atmosphere of the refractory metals has led to the development of Low-Temperature-Cofired-Ceramic (LTCC) tape. LTCCs are under development and/or commercially available from a number of manufacturers including Electro-Science Laboratories, Inc., of Prussia, Pa., EMCA, of Montgomeryville, Pa., and FERRO, of Santa Barbara, Calif. A preferred LTCC, which is known in the art as "green tape", is commercially available from the DuPont Company under the product designation #851AT. The tape contains a material formulation which can be a mixture of glass and ceramic fillers which sinter at about 850.degree. C., and exhibits thermal expansion similar to alumina.
The low-temperature processing permits the use of air fired resistors and precious metal thick film conductors such as gold, silver, or their alloys. In the typical high-temperature process, screen-printed resistors cannot be used and only refractory metal pastes are used as conductors.
A discussion of thick film technology, and high and low temperature cofired ceramic tape technology, is found in "DEVELOPMENT OF A LOW TEMPERATURE COFIRED MULTILAYER CERAMIC TECHNOLOGY", by William Vitriol et al, ISHM Proceedings 1983, pp. 593-598.
One disadvantage of the cofired ceramic approach is that the dielectric film or tape will undergo shrinkage of as much as 20% in each of the X, Y, and Z directions. This shrinkage results in a dimensional uncertainty in the fired part of typically 1%, which may be unacceptable in the fabrication of certain types of hybrid microcircuits.
The third prior art multilayer circuit board fabrication technology is disclosed in U.S. Pat. No. 4,645,552, issued Feb. 24, 1987, entitled "PROCESS FOR FABRICATING DIMENSIONALLY STABLE INTERCONNECT BOARDS", to William Vitriol et al. This process may be described as a "transfer-tape" method, and is performed by providing a generally rigid, conductive substrate, or an insulative substrate on which a conductive circuit pattern is formed, and then transferring and firing a glass-ceramic tape layer to the surface of the substrate. This tape layer provides electrical isolation between the substrate and electrical conductors or electronic components which are subsequently bonded to or mounted on the top surface of the glass-ceramic tape layer. By providing vertical electrical interconnects by means of vias formed in the tape layer prior to firing the tape layer directly on the substrate, good X and Y lateral dimensional stability of the tape material is maintained. The next conductor layer in this vertical interconnect process is then screen printed on the fired tape dielectric and itself fired. This process is repeated until the hybrid circuit is built up to a desired vertical, multilayer interconnect level. As an alternative process to individually firing conductor and dielectric layers, the complete structure or portions thereof can be simultaneously fired as disclosed in the above referenced patent to Vitriol. By replacing a screen printed dielectric layer build-up process with a pre-punched dielectric tape layer, the transfer tape process retains the primary advantages of the thick film process, while gaining many advantages of the cofired ceramic process.
All three of the processes described above produce essentially flat, or planar, circuit structures. The thick film and transfer processes, in particular, utilize relatively rigid substrates which are, by their nature, flat.
Known processes for forming edge connectors on microelectronic circuit boards generally include depositing or otherwise forming conductive strips directly on the edge portions of the boards. This requires at least one process step which is additional to forming intercomponent metallizations and electrical components on the boards.